Chlorosulfuric acid

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Chlorosulfuric acid
IUPAC name
Sulfurochloridic acid
Other names
Chloridosulfonic acid
Chloridosulphonic acid
Chlorinesulfonic acid
Chlorinesulphonic acid
Chlorosulfonic acid
Chlorosulphonic acid
Sulfuric chlorohydrin
Molar mass 116.52 g/mol
Appearance Colorless liquid, fuming
Odor Pungent, acidic
Density 1.753 g/cm3 (20 °C)[1]
Melting point −80 °C (−112 °F; 193 K)
Boiling point 151–152 °C (304–306 °F; 424–425 K)
Solubility Reacts with alcohols, bases, esters, ketones
Soluble in acetic acid, acetic anhydride, halocarbons, liq. HCl, nitrobenzene, liq. SO2, sulfuryl chloride, trifluoroacetic acid
Sparingly soluble in carbon disulfide, CCl4
Insoluble in hydrocarbons
Vapor pressure 1 mmHg (20 °C)
-555.2 kJ/mol
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Related compounds
Related compounds
Sulfuric acid
Fluorosulfuric acid
Sulfuryl chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Chlorosulfuric acid, also called chlorosulfonic acid or sulfurochloridic acid is the inorganic compound with the formula HSO3Cl.

Salts and esters of chlorosulfuric acid are known as chlorosulfates.



Chlorosulfuric acid reacts violently with water to yield sulfuric acid and hydrogen chloride, commonly seen as vapors fuming copiously from the liquid:

ClSO3H + H2O → H2SO4 + HCl

Above its boiling point, chlorosulfonic acid decomposes, releasing sulfur trioxide.

Reaction with solid potassium permanganate yields permanganyl chloride (MnO3Cl).

It also reacts with acetone, yielding a reddish liquid, with a unique odor.

Chlorosulfuric acid ignites lithium aluminium hydride on direct contact.

Reactive metals, like magnesium, and even alkali metals like sodium or magnesium react poorly with chlorosulfonic acid.This is due to the lack of water in the acid, as adding a few drops of pure water will cause the reaction to speed up significantly. However, some metals, like tin, react with chlorosulfuric acid to yield fumes of tin(IV) chloride.[2]

Reaction with nonmetals an metalloids give different products:

Reaction with selenium yields a green product, which is Se82+. Heating this solution will cause it to turn yellow, as the Se82+ is turned into Se42+.

Iodine dissolves in chlorosulfonic acid to yield an ionic pentaiodide compound.

Reaction with nitrates, like potassium nitrate, yields nitrogen dioxide and chlorine gas, while with silver nitrate it reacts violently, yielding silver chloride and nitroxylchlorosulfonate (NO-O-SO2Cl) fumes.

Chlorosulfuric acid will react with dry sodium chloride to yield sodium chlorosulfonate.

HSO3Cl + NaCl → NaSO3Cl + HCl

An early synthesis of saccharin involved the reaction of chlorosulfuric acid with toluene to give the ortho- and para-toluenesulfonyl chloride derivatives.


Chlorosulfuric acid is a fuming, colorless liquid (though commercial samples usually are pale brown or straw colored), which is hygroscopic and a powerful lachrymator. It reacts with water and alcohols and protic solvents, but dissolves in halocarbons.


Chlorosulfuric acid is sold by chemical suppliers, though due to its hazards, it's not easy to acquire.


There are several routes to chlorosulfonic acid, all of which are very dangerous and must not be done by the hobby chemist unless they have very good ventilation, either inside or outside, and proper chemical protection.

A simple high yield route involves the distillation of phosphorus pentachloride with conc. sulfuric acid.[3]

PCl5 + H2SO4 → HSO3Cl + POCl3 + HCl

The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid aka oleum:

HCl + SO3 → HSO3Cl

Reaction of dry chloroform with sulfur trioxide yields chlorosulfuric acid, pyrosulfuryl chloride and carbon monoxide:[4]

CHCl3 + 3 SO3 → HSO3Cl + S2O5Cl2 + CO


  • Make sodium chlorosulfonate
  • Make alkyl sulfates
  • Synthesis of saccharin
  • Anti-contrail agent



Chlorosulfonic acid fumes in open air, releasing HCl and sulfuric acid fumes. These are highly corrosive and thus proper protection, like gas mask, thick chemical-resistant gloves and full body covering must be worn when handling the compound. Even in open air, it's difficult to properly handle it without some form of protection.[5]


Chlorosulfonic acid must e kept in glass bottles with an hermetic seal/cap, preferably with PTFE seal. To prevent any fumes, it can be kept in a plastic container, filled with an absorbant material, to reduce any leaking corrosive fumes.


It is difficult to safely dispose of this compound. One way is to take a large container, where lots of crushed ice is added, and the acid is added inside in a controlled manner, like dropwise, all while the container is kept closed to limit the fumes escaping outside.


  1. Sanger, C. R.; Riegel, E. R.; Z. Anorg. Chem.; vol. 76; (1912); p. 79 - 128
  2. Chlorosulfonic acid. Powerful lachrymator ⚠
  3. Distilling A Superacid In My Garage: Chlorosulfonic Acid Synthesis and Reactions
  4. Gilbert, E. E.; Chemical Reviews (Washington, DC, United States); vol. 62; (1962); p. 549 - 589
  5. Superacid vs. Everything

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